1. FIELD OF THE INVENTION
This invention relates to a process for preparing fine acicular .alpha.-FeOOH particles, i.e., having a large specific surface area, sharp particle size distribution and, in particular, improved acicularity, such acicular .alpha.-FeOOH being intended to serve as a precursor for obtaining ferromagnetic iron oxides which are suitably used as a magnetic recording medium having a low noise characteristic, excellent coercivity and other magnetic properties.
2. DESCRIPTION OF THE PRIOR ART
In relation to the correlation between particle size reduction and product quality, it is known that an improvement of the S-N ratio (S/N) can be attained by increasing the number of the particles in the magnetic medium by means of particle size reduction (see Journal of Audio Engineering Society, Vol. 20, pp. 98-99 (1972)) and that an improvement of the S-N ratio (S/N) can be also realized by the volume reduction of individual particles (see IEEE Transaction on Magnetics, Vol. Mag. 17, No. 6, pp. 3032-3034 (1981)). However, such particle size reduction is attended by various problems, such as (1) lowering of acicularity (axial ratio) due to decrease of a major axis length, (2) lowering of heat resistance of particles in the step of heat treatment, and (3) occurrence of branching and encouraged formation of new nuclei when a growth factor is elevated for increasing the yield in industrial practice, and thus improvements over these problems have been desired.
For the preparation from a ferrous salt of acicular .alpha.-FeOOH intended to be used as a precursor for obtaining acicular ferromagnetic iron oxides, there are known roughly the two following types of methods: an acid method in which the .alpha.-FeOOH is produced by the precipitation in an acidic region and an alkaline method in which the .alpha.-FeOOH is precipitated in an alkaline region. The acidic method is advantageous over the alkaline method in that the former is subject to less restrictions in starting materials as the impurities contained in the starting material ferrous salt are discouraged from being coprecipitated during the reaction and that the former is more economical because of less alkali consumption but, on the other hand, the former involves such problems that the produced .alpha.-FeOOH particles might prove unsatisfactory in axial ratio (length/width ratio) and that the particle size distribution tends to be broadened. The axial ratio tends to lower particularly when the preparation reaction is carried out at a temperature above 45.degree. C. Improvement of the axial ratio of the .alpha.-FeOOH particles leads to betterment of coercivity due to the geometric anisotropy of the particles as well as improvement of other magnetic properties when a magnetic coating film is produced by using a ferromagnetic iron oxide obtained from said .alpha.-FeOOH in a known way, and thus improvements of such axial ratio and particle size distribution have been strongly desired, especially in the acidic method. Many attempts have been made hitherto for the improvement of axial ratio. For instance, Japanese Patent Publication No. 49016/80 proposes the use of .gamma.-FeOOH with better acicularity in place of .alpha.-FeOOH, while Japanese Patent Publication No. 23217/80 teaches the addition of Zn ions in the course of formation of .alpha.-FeOOH seed crystals, and Japanese Patent Laid-Open No. 127400/78 suggests the preparation of .alpha.-FeOOH under a highly alkaline condition. However, these methods, though capable of improving the axial ratio, can hardly be deemed as enough improvements for such reasons that the improvements of other properties are unsatisfactory and the problems are left unsolved in their application to industrial production.
Also, Japanese Patent Publication No. 25546/64 discloses a process in which the precipitation and oxidation of ferrous hydroxide are performed in the presence of phosphate ions at a temperature of up to 50.degree. C. to form the seed crystals and then metallic iron and an oxidizing agent are added to a suspension of said seed crystals to grow the particles of said seed crystals to the extent of reaching 6 to 30 times greater than the total weight thereof to thereby obtain rather stumpy short columnar .alpha.-FeOOH particles. In this process, however, only orthophosphoric acid is used as a phosphoric acid and no improvement is provided in the axial ratio of particle.